The present invention relates to a servo mark detection device for detecting a servo mark pattern in a magnetic disk apparatus of a data surface servo scheme.
In a magnetic disk apparatus of the data surface servo scheme, an area for recording a servo pattern and position information such as a sector address and a cylinder address, which are used to position a magnetic head, is set on the same surface as the data surface of a magnetic disk medium. A data pattern called a servo mark indicating the start position of the position information recording area is recorded on the data surface. As a servo mark detection circuit for detecting this servo mark, an arrangement like the one shown in FIGS. 8 and 9 has been used.
In the apparatus, the above data pattern is read by a head, and the resultant signal is input to a circuit such as a peak detector. The timings of the peaks of the amplitude of the signal are detected by this circuit. As a result, the signal is shaped into a raw read data pulse signal (to be referred to as an RRD signal hereinafter) a.
The conventional servo mark detection circuit includes a synchronizing circuit 1 for receiving this RRD signal a and outputting a synchronization data signal b synchronized with the apparatus clock, a shift register circuit 2 for shifting/storing the synchronization data signal b and outputting an N-bit latch data signal c, and a servo mark comparator 10 for receiving the latch data signal c, checking whether the signal coincides with a pattern corresponding to a predetermined servo mark, and outputting a detection signal m indicating that the servo mark has been detected.
As shown in FIG. 9, the servo mark comparator 10 includes a plurality of comparators 11 to 14 for commonly receiving the latch data signal c and an OR circuit 15 for receiving outputs 101 to 104 from the comparators 11 to 14 and outputting the detection signal m. Although FIG. 9 shows only four comparators 11 to 14 for the sake of descriptive convenience, an actual circuit requires several ten or more comparators.
The operation of the conventional servo mark detection circuit having the above arrangement will be described next with reference to FIGS. 10A to 10E. FIG. 10E shows an example of the servo mark pattern. FIG. 10A shows the RRD signal a corresponding to the servo mark pattern. When this RRD signal a is frequency-divided in the synchronizing circuit 1, the frequency-divided signal shown in FIG. 10B can be obtained. FIG. 10D shows the synchronization data signal b obtained by inputting the signal to the synchronizing circuit 1. The signal b is an output obtained from the synchronizing circuit 1 when the RRD signal a has an ideal waveform without any jitter.
In practice, since the RRD signal a and the apparatus clock shown in FIG. 10C are asynchronous, and fluctuations are caused in both the waveforms owing to variations in rotation in the apparatus, variations in power, and the like, jitter is caused in the synchronization data signal b. For this reason, the synchronization data signal b output from the synchronizing circuit 1 does not always have the waveform shown in FIG. 10D, but may vary forward/backward on the time axis. According to the conventional circuit, therefore, in the servo mark comparator 10 for detecting a servo pattern, forward/backward shifts on the time axis must be allowed within a certain limit, and all possible patterns must be prepared as reference patterns, thereby checking whether a detected pattern coincides with one of the reference patterns.
For this operation, complicated comparators are required, and the operation of each comparator cannot be guaranteed as the clock frequency increases owing to element delays. In addition, to compare a detected pattern with all possible patterns, an enormous number of comparators are required. The apparatus cannot therefore be implemented in practice. For this reason, the number of patterns must be determined by making a compromise. Consequently, a decoding error may occur.